The present invention relates to a container for optical components. In particular, active and/or passive optical components such as, for example, laser sources, optical splitters, components made with integrated optics and fibre devices can be contained inside a container. In general, for the purposes of the present invention, optical components are considered as being passive or active optical device, for example electrooptic and/or acousto-optic devices which require one or more connections with optical fibres.
In general, these components suffer from contact with particles of water contained in humidity and with other atmospheric agents, for example corrosive agents, which can give rise to a decline in the performance of the component, up to the point at which its correct functioning is compromised. In addition, in certain components, for example lasers, it is necessary-that a defined gas composition inside the container remain unaltered for reasons of reliability.
Examples of optical components which require sealing are, for example, electrooptic modulators made on lithium niobate substrates, and acousto-optic devices. The materials inside the container which suffer most from contact with humidity are, for example, the adhesives used to attach the components and the fibres inside the container.
Thus, what is required is containers which have good sealing, allowing the environment inside them to be kept free of humidity and of other external agents, thus allowing all the components present therein to function correctly. In general, these containers are made of a metallic material, for example stainless steel, Kovar(trademark), metal alloys such as copper-tungsten, for example, and, more generally, materials which do not allow humidity to penetrate into the container, The points at which the passage of humidity or of other agents into the container can take place are the regions in which the electrical and optical connections, referred to hereinbelow as electrical feedthroughs and optical feedthroughs, respectively, with the components present inside the container are located.
Another region in which humidity or other external agents can penetrate into the container, when the said container comprises a base and a lid, is the region of the closure between the lid and the base. In order to ensure sealing in this case, the lid is generally closed using a known technique of brazing along the entire peripheral region of contact. This brazing is carried out, for example, by subjecting the said region of contact to an electrical discharge. The metallic material both of the lid and of the base in the said region fuses and hermetically fixes the two parts together.
For the passage of optical fibres into the container from the outside, the optical feedthroughs are sealed using various techniques.
An optical fibre has a portion made of glass with a core and cladding; generally, the dimensions (diameter) are about 10 xcexcm for the core and about 125 xcexcm for the cladding for monomodal optical fibres. In all the types of fibre, the said portion made of glass has a coating made, for example, of acrylate (acrylic resins) or of elastomeric material in general, which has a minimum diameter of about 250 xcexcm and a maximum diameter of about 400 xcexcm in the case of fibres with maintained polarization. A description of fibres with maintained polarization is given in patent application 98EP-100185.2 in the name of the Applicant.
This coating has pores such that they allow humidity to pass into it and thus, when this coating is not removed in making a feedthrough, they also allow humidity to pass into the container and contact the various optical components. The only portion of the optical fibre which ensures sealing is the portion made of glass, which does not allow any passage of humidity or of other agents.
Thus, in order to seal an optical feedthrough, it is necessary to remove the fibre""s acrylate coating in the region which will be inserted through the optical feedthrough, thus exposing the xe2x80x9cnakedxe2x80x9d fibre, i.e. the portion made of glass only.
A technique which effects the sealing involves metallizing the fibre, i.e. coating the naked fibre with a metallic layer which is in direct contact with the glass.
An example of metallization of a fibre is described in U.S. Pat. No. 4,779,788, in which the metallization process takes place after the outer surface of the fibre has been cleaned thoroughly by immersing it, for example, in hot sulphuric acid and washing it with deionized water. The subsequent metallization takes place by means of a process of evaporating metal particles, which settle on the glass of the fibre and become firmly attached thereto, without allowing any humidity to pass between the fibre and the metal.
An example of this technique for sealing an optical feedthrough is described in European patent application EP 690,322, which describes a method for generating a sealed feedthrough, in which a length of the fibre is first stripped of its acrylate coating. The fibre is placed in a cylindrical body having a first portion in which is placed the length of metallized fibre and a second portion, which has a larger diameter than the first portion, in which is placed the length of fibre following the section stripped of coating. The portion of metallized fibre is soldered inside the said first portion of the cylindrical body using a metallic soldering alloy and the portion of fibre with the coating is attached in the second portion of the cylindrical body by means of an adhesive. Next, the cylindrical body, with the fibre attached inside it, is placed in a feedthrough hole appropriately made on a wall of the container and is fixed therein by soldering around the circumference of the feedthrough hole.
The Applicant has found that this technique is expensive, mainly due to the preparation of the metallized fibre, which is a particularly intricate process, since the naked fibre is very fragile and can thus be damaged during the metallization. In addition, the process is carried out using complex machinery. The metallization must be carried outside the container and before the fibre is connected to any of the components housed in the container.
Another technique for sealing an optical feedthrough is described in U.S. Pat. No. 5,177,806, which relates to an optical feedthrough in which the fibre, stripped of the acrylate protective layer, is soldered into a tube by means of glass powder (xe2x80x9cglass solderxe2x80x9d). In particular, that patent describes an optical feedthrough in which a fibre is maintained in a fixed position in a metal sleeve, while a glass powder is brought to high temperature and then cooled to form a solder inside this sleeve on the naked fibre.
The Applicant has observed that, although this solution eliminates the problems associated with metallization of the fibre, it introduces a high risk of damage to the components inside the container. In fact, the glass powders have relatively high melting points (350xc2x0 C.-500xc2x0 C.) and the components generally must not be taken up to such high temperatures. In this case also, it is necessary to carry out the soldering inside the sleeve before connecting the fibre to the components and mounting these components in the container.
When the fibres are of the type with maintained polarization, the xe2x80x9cglass solderxe2x80x9d in contact with the said fibre can bring about a reduction of the polarization extinction ratio of this fibre.
Another technique for sealing an optical feedthrough, described in patent application EP 469,853, involves using an epoxy resin to prevent humidity from passing into the feedthrough.
In particular, that patent application describes a light guide with an optical fibre for guiding light from an external source into a closed space. The optical fibre is placed in a feedthrough hole formed in a fixing support for the said light guide. The said support has an inlet hole which communicates with the said feedthrough hole. The optical fibre is coated with a surface layer throughout the feedthrough hole, except for the portion which communicates with the said inlet hole, in which it is naked. An epoxy resin is injected into the inlet hole and comes into contact with the fibre and completely occludes the empty spaces, i.e. in the region in which the fibre is free of coating and along the inlet hole.
The support comprises means for fixing it to a wall of the said closed space, which has a communication hole in which the said support is placed and fixed.
The Applicant has observed that, although epoxy resins are highly leaktight, they however have a porosity which, over a long period of time, in environments with a high humidity content and high temperatures (above 30xc2x0), can bring about entry of humidity into the container. In addition, epoxy resins have-relatively high crosslinking temperatures and coefficients of thermal dilation which are greater than those of optical fibres, which can result in mechanical-type stress on these fibres.
The Applicant has found that one parameter which has an influence on the leaktightness of a feedthrough is the space between the glass portion of the fibre and the hole which needs to be filled with the material for sealing this feedthrough. Thus, by using resin in contact with the fibre the leaktightness and the resistance to humidity of the feedthrough are inversely proportional to the cross-sectional area to be filled. In order to increase the leaktightness, it is necessary to reduce the inside diameter of the feedthrough down to the smallest dimensions of the fibre. The diameter of the fibre with the acrylate coating can be up to about 400 xcexcm and this measurement thus represents the lower limit for the inside diameter of the feedthrough hole.
The Applicant has found that a combined technique which involves using a metallic soldering alloy on naked fibre to reduce the diameter of the feedthrough and a polymeric sealant in order to carry out the operation for sealing this feedthrough, seals the feedthrough hermetically without compromising the properties of the fibre. This technique allows the polymeric sealant to come into contact with the fibre only in a limited region of the feedthrough, a few tenths of a millimeter, thus resulting in a reduction of the stresses on the fibre. Moreover, by minimizing the area of the circular section filled with polymeric sealant, better sealing is obtained. In addition, the operations required to seal the feedthrough can be carried out after connecting the fibre to the component itself and mounting this component in the container, as the final operation before closing the container.
A first aspect of the present invention relates to a protective system for optical components, comprising:
a container,
at least one optical component fixed inside the said container,
a length of optical fibre which has a plastic coating and is connected to the said optical component, and
an optical feedthrough for the said section of fibre, placed in a feedthrough hole in a wall of the said container and hermetically fixed therein,
the said optical feedthrough comprising an elongate body which has a longitudinal feedthrough hole into which the said optical fibre can be placed,
a portion of the said length of fibre being stripped of the said coating,
characterized in that
the said portion of fibre which is stripped of the coating is soldered to one end of the said elongate body by means of a metallic solder, and this soldering, in the surface portion around the said portion of fibre, is coated with a layer of a polymeric sealant.
Preferably, the said polymeric sealant is an epoxy resin.
In particular, the end of the said elongate body at which the soldering is carried out has a surface which is at an angle of less than 90xc2x0 relative to the base of the container.
Preferably, the said angle is comprised between 30xc2x0 and 60xc2x0.
In particular, the said elongate body is cylindrical and has a rear portion with an outside diameter which is slightly smaller than the diameter of the hole made in the wall of the container, and a front portion with an outside diameter which is smaller than the diameter of the rear portion.
Preferably, at least one transverse notch communicating with the feedthrough hole is present in the said rear portion.
Preferably, the said fibre is a fibre with maintained polarization.
A further aspect of the present invention relates to a sealed feedthrough for optical fibres, comprising
an elongate body with a longitudinal feedthrough hole in which can be placed an optical fibre provided with a plastic coating,
a portion of the said fibre (F) being stripped of the said coating, characterized in that
the said stripped portion of the fibre is soldered to one end of the said body by means of a metallic solder, and this soldering, in the surface portion around the said portion of fibre, is coated with a layer of a polymeric sealant.
Preferably, the said polymeric sealant is an epoxy resin.
In particular, the end of the said elongate body at which the soldering is carried out has a surface at an angle of less than 90xc2x0 relative to the longitudinal axis of the said elongate body.
Preferably, the said angle is comprised between 30xc2x0 and 60xc2x0.
In particular, the said elongate body is cylindrical and has a rear portion with a first outside diameter, and a front portion with an outside diameter which is smaller than the said first diameter of the rear portion.
Preferably, at least one transverse notch communicating with the said feedthrough hole is present in the said rear portion.
A further aspect of the present invention relates to a method for protecting optical components, comprising the following steps:
connecting a length of optical fibre to an optical component,
mounting the said component inside a base of a container,
placing an optical feedthrough in a hole present in a wall of the base,
hermetically fixing the said feedthrough in the said wall,
stripping a portion of the said optical fibre of its plastic coating,
sealing the said feedthrough,
hermetically closing the container with a lid,
characterized in that the said step of sealing the said feedthrough comprises the steps of
threading the free end of the said fibre into a hole in the said feedthrough,
soldering, with a metal alloy, the said portion of fibre stripped of the said coating, to the end of the feedthrough which is placed inside the container, and
placing a layer of a polymeric sealant in the surface portion of the said solder around the said fibre.
Preferably, the step of mechanically fixing the fibre to the inside of the said feedthrough occurs between the steps of threading the free end of the fibre into a hole in the feedthrough and the step of soldering the stripped portion of fibre with a metal alloy.
A further aspect of the present invention relates to a method for sealing an optical feedthrough which connects, by means of an optical fibre, an optical component placed inside a container with the outside, characterized in that it comprises the following steps:
stripping a portion of the said optical fibre of a layer of plastic coating,
threading the free end of the said fibre into a hole in the said feedthrough,
mechanically fixing the fibre inside the said feedthrough,
soldering, with a metal alloy, the said portion of fibre stripped of the said coating, to the end of the feedthrough placed inside the container,
placing a layer of a polymeric sealant in the surface portion of the said solder around the said fibre.
A further aspect of the present invention relates to a method for hermetically sealing an optical fibre with respect to a feedthrough, characterized in that it comprises the following steps:
stripping a portion of the said fibre of the coating so as to expose the glass,
filling the space between the fibre and the feedthrough with a soldering alloy,
sealing with a polymeric sealant the portion of soldering alloy in contact with the glass of the fibre.